Reduction of zirconium and hafnium tetrachlorides with liquid magnesium



June 16, 1964 H. RENNER 3,137,568

REDUCTION OF ZIRCONIUM AND HAFNIUM TETRACHLORIDES WITH LIQUID MAGNESIUM Filed May 31, 1961 2 Sheets-Sheet 1 l INVENTOR //PM/VA//P/V/VEP,

ATTORNEYS June 16, 1964 H. RENNER 3,137,568

REDUCTION OF ZIRCONIUM AND HAFNIUM TETRACHLORIDES WITH LIQUID MAGNE IUM Filed May 51, 1961 2 Sheets-Sheet 2 ATTORNEY5 \zzy United States The present invention relates to an improved process for the reduction of zirconium and hafnium tetrachlorides to produce the corresponding metals.

In general, the production of highly pure, ductile zirconium is usually effected by the process of W. Kroll by reduction of ZrCl, with Mg to Zr sponge. The Kroll reaction proceeds between gaseous ZrCl, and melted Mg. The apparatus employed therefore is a reaction vessel of stainless steel containing a crucible for the Mg supply in which the reduction reaction proceeds and an arrangement thereover for supplying ZrCl All gases reactive with zirconium metal are displaced in the reaction vessel by an inert gas such as argon. The reaction vessel is heated from the exterior to cause the ZrCl to vaporize and the Mg to fuse. The vaporized ZrCl, diffuses to the Mg and the reaction takes place at the interface between the vaporized ZrCl and the liquid Mg. Subsequently the MgCl which is formed is separated from the Zr by distillation under high vacuum.

The progress of the reaction is essentially determined by the velocity of the vaporization of the ZrC1 The regulation of the velocity of such vaporization engenders n occurs in such gas space. The ZrCl formed outside of the reduction crucible is very undesirable. In general it contains substantial quantities of Zr derived from the starting material and therefore reduces the yields of Zr metal considerably. The spontaneous ignition of the substance can during discharge easily also ignite the Zr metal produced and therefore endanger the entire charge. Furthermore, the black dust is very disagreeable to handle and when it burns large quantities of HCl fog are produced. Also, when the reaction temperature is raised too high, the eutectic melting point of the ZrFe system (934 C.) is easily reached and the zirconium produced alloys with the iron of the crucible wall. Zr containing more than 0.1% Fe is no longer usable as reactor Zr.

The control of the reaction by the correct adjustment of the ZrCL; vapor pressure is very difficult and is only possible to a somewhat satisfactory extent with considerable attention by the operating personnel. In addition, considerable demands are made on the skill of the operating personnel. If the desired reaction rate is exceeded it is most diflicult to reduce such rate by the proper amount,

Usually the countermeasures taken to reduce the rate of reaction cause it to cease almost entirely. In many instances it is almost impossible to recognize the formation of ZrCl or respectively an increase in temperature promptly, namely, when the distribution of the MgCl produced in the reaction alters the surface area of the Mg melt quickly or pulsatingly as often occurs and locally limited temperature rises and Mg escapes occur.

According to the invention it was unexpectedly found that the undesired side reaction could be completely and reliably eliminated by carrying out the reducing reaction at the interface between molten magnesium and a melt of a halide salt having ZrCL; dissolved therein. The zirconium containing halide salt melt (halogen zirconate) is formed separately from the reduction of the quadrivalent Zr therein to the metal. Preferably, the halogen zirconate is produced at a temperature below the melting point of Mg at a different time than that of the reduction reaction. It is expedient thereby to carry out the zirconate production and the reduction in the same crucible and repeating the sequence of such steps without changing the molten salt bath.

The reduction reaction is transferred from the interface between ZrCl vapor and molten Mg to below the surface of the molten salt bath by first permitting the ZrC1 vapor to diffuse into the molten salt bath and then carrying out the reduction of the Zn in the molten salt bath by the molten Mg, the supply of ZrCl vapors to the molten salt bath being terminated before the reduction reaction is carried out. Both steps are preferably repeated alternately with a single batch. Salt like halides which, at least in the molten state, are capable of dissolving ZrCl, at temperatures below the melting point of Mg are suited for the production of the molten salt bath. Preferably NaCl or KCl or NaF can be used.

In the accompanying drawings:

FIG. 1 diagrammatically shows an apparatus suitable for carrying out the process according to the invention; and

FIGS. 2:: through 2e schematically show the progress of the process according to the invention upon alternate repetition of the steps of such process.

In the apparatus shown in FIG. 1, a reaction vessel 2 is arranged within furnace 1. Two superposed metal crucibles 3 and 4 are housed within such reaction vessel 2 with a small space therebetween. The furnace is provided with two heating coils 5 and 6 which permit independent regulation of the reaction temperatures in the upper and lower ends of vessel 2. The temperatures are either controlled automatically or by thermocouples 7 and 8 over regulators 9 and 10. Lower crucible 4 is provided with a covering plate 20 having an opening which is closed with a lid 21 which can be operated from the exterior of the furnace.

To carry out the process according to the invention 45 kg. of ZrCl are placed in the upper crucible and 11.3 kg. of magnesium and 10 kg. of solid NaCl in the lower crucible. After all of the air has been displaced from the reaction vessel with argon or helium, the contents of crucible 4 are heated to 600 C. and then the upper crucible 3 is heated to vaporize ZrCl, to supply ZrCl, vapors to crucible 4 until saturation is achieved. As the system NaCl/ZrCL; has a melting point considerably below the melting point of NaCl (800 C.) with even only small contents of ZrCl for example, at a ZrCl content its melting point is 400 C, a liquid melt spreads out over the magnesium which has not yet melted. As soon as this melt is saturated with ZrCl the ZrCl vaporization is terminated and the reducing crucible 4 closed by application of lid 21. Thereupon the reduction is initiated by heating the lower zone to 800 C.

FIGS. 2ae illustrate schematically the stepwise progress of the process according to the invention. Such figures only show the lower crucible 4 which in the first step (FIG. 2a) contains a melt of sodium chlorozirconate which forms over magnesium metal pigs 12. When such crucible is then heated to over 800 C., a first reduction occurs with the formation of metallic Zr 13 over which after cooling to 600 C. metallic magnesium 14 deposits, the hollow space of which contains the salt melt consisting of NaCl and the MgCl formed in the reduction (FIG. 2b). Upon further supply of ZrCL; vapor to such salt melt a sodium chlorozirconate melt 16 of increased volume is formed over such magnesium 14 and zirconium 13 (FIG. When the temperature of the crucible is again raised to over 800 C. a further reduction is effected with the production of more zirconium which deposits at the bottom of the crucible, over which metallic magnesium 18 and a salt melt 19 consisting of NaCl and MgCl lie (FIG. 2d). The small remainder of the metallic magnesium can be reacted to completion by simultaneous sodium chlorozirconate formation and reduction (FIG. 2e). With the quantities of starting materials indicated above 17 kg. of Zr sponge are produced.

The reduction process according to the invention wherein the formation of NaCLZrCL; is chronologically spaced from the actual reduction provides for a number of advantages over the original Kroll process and its known variations as can be seen from the following:

(a) The formation of ZrCl as an undesired by-product is completely and reliably eliminated by effecting the reduction under the surfaceof the molten salt. Previously considerable skillful attendance of operating personnel was required to prevent ZrCl formation.

(b) The regulation of the progress of the process is effected by following clear operating instructions and is not dependent upon subjective decisions and senses of operating personnel. Only very few regulating controls are required, the supervision and operation of which are simple. Above all, the repeated blowing off of ZrCl vapor which was often necessary in the Kroll process is considerably limited. 7

(c) The presence of alkali metal halide produces an acceleration of the overall reaction, above all, because even at the end of the reaction the ZrCl as alkali metal halozirconate can react the Mg residues and the Mg need not exclusively diffuse to the surface of the melt through the diffusion hindering Zr sponge produced. This advantage renders it possible to reduce the previously necessary high and costly Mg excess. For the same reason, in contrast to the Kroll process, all of the ZrCl supplied can react to completion with no residue remaining.

(d) A more favorable spatial distribution of the reaction heat occurs in the reaction between the alkali metal halozirconate with the magnesium than in the reactionof ZrCL, vapor at the surface of the Mg melt. The reaction which occurs under the surface of the salt melt does not concentrate near the Wall of the iron crucible but rather is evenly distributed over the entire interface between the salt melt and Mg melt so that the formation of alloys between Zr and Fe is reduced considerably. In the surface reaction between ZrCl vapor and the Mg melt the reaction concentrates at the crucible wall as a thin layer of liquid Mg creeps up the wall by capillary action. This is also shown by the form of the Zr sponge deposit. In the ZrCl vapor/ Mg melt reaction the sponge for the greatest part grows on the side walls of the crucible, in the process according to the invention it primarily is deposited on the bottom plate which is first formed. The removal of the Zr sponge produced according to the invention from the crucible is easier.

(e) The heat of reaction of ZrCl +2Mg- Zr+2MgCl +74 kcal.

is distributed in the two sequential reactions ZrCl +NaCl NaZrCl /s x 74 kcal.

and

This causes a favorable time distribution of the heat of reaction which again counteracts the possibility of temperature rises by concentrations of the heat of reaction.

(f) In all commercial variations of the Kroll process there is the danger of occurrence of sharp temperature peaks because of sudden activation of the reaction, for example, as a result of increase in the ZrCL; supplied or freeing of Mg surface. It is therefore never possible to operate near the favorable upper limits with regard to reaction velocity and temperature in order that the above-.

duced under a salt melt so that there is less tendency to take up impurities from the atmosphere of thereaction vessel than a material directly in contact with the gas phase.

(h) The NaCl/MgCl mixture is much less hydroscopic than pure MgCl than actually could be expected. As a consequence the possibility of water, which later is formed as oxygen in the. metal, being absorbed before the salt is distilled off from the Zr sponge is materially reduced.

The process according to the invention while having been describedwith reference to the production of zirconium is equally well suited in all of its details for the production of hafnium.

Hafnium and zirconium are, as known, in all chemical properties of the same kind. The zirconium obtained from minerals contains 1 to 10% of hafnium. The metal chlorides of both metals may be reduced together or after a separation by known processes. The metallic sponge produced according to the invention is very pure. The content of O is lower than 400 p.p.m. of Fe lower than 300 p.p.m., of C lower than 40 p.p.m. and of N lower than 30 p.p.m. The Brinell-hardness lies by HB.

Instead of the described use of NaCl also other halogenides, e.g. KCl and NaF, are suitable for use single or in mixtures. lies below the density of Mg and Zr (NaCl: 1.5, Mg: 1.74) so that the metals always deposit at the bottom of the crucible.

- I claim: a

1. A process for the production of zirconium metal which comprises placing solid magnesium covered by an alkali metal halide in a reaction vessel, contacting ZrCl vapors with said alkali metal halide at a sufliciently raised temperature below the melting point of the magnesium that ZrCl; is dissolved by said alkali metal halide to form a fused alkali metal halide solution of said ZrCl over said magnesium metal while said magnesium metal is solid, raising the'temperature of the reaction vessel to fuse the magnesium in contact with the supernatant fused alkali metal halide solution whereby the ZrCl; contained in such solution is reduced to Zr sponge at the interface between the liquid magnesium and such supernatant solution.

The density of the salts in the molten state' 2. The process of claim 1 in which said alkali metal halide is NaCl and the formation of the ZrC1 solution therein is effected at about 600 C. and the reduction is effected at about 800 C.

3. The process of claim 1 in which the quantity of magnesium placed in the reaction vessel is greater than will react with the ZrCl dissolved in the alkali metal halide melt comprising in addition after completion of the reduction of the dissolved ZrCl reducing the temperature of the reaction vessel below the melting point of the remaining magnesium, contacting further ZrC1 with the remaining salt melt at a temperature below the melting point of the magnesium to absorb further quantities of ZrCl therein,

and raising the temperature of the reaction vessel over the melting point of the magnesium whereby the fused magnesium reacts with the further quantities of absorbed ZrCl References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR THE PRODUCTION OF ZIRCONIUM METAL WHICH COMPRISES PLACING SOLID MAGNESIUM COVERED BY AN ALKALI METAL HALIDE IN A REACTION VESSEL, CONTACTING ZRCL4 VAPORS WITH SAID ALKALI METAL HALIDE AT A SUFFICIENTLY RAISED TEMPERATURE BELOW THE MELTING POINT OF THE MAGNESIUM THAT ZRCL4 IS DISSOLVED BY SAID ALKALI METAL HALIDE TO FORM A FUSED ALKALI METAL HALIDE SOLUTION OF SAID ZRCL4 OVER SAID MAGNESIUM METAL WHILE SAID MAGNESIUM METAL IS SOLID, RAISING THE TEMPERATURE OF THE REACTION VESSEL TO FUSE THE MAGNESIUM IN CONTACT WITH THE SUPERNATANT FUSED ALKALI METAL HALIDE SOLUTION WHEREBY THE ZRCL4 CONTAINED IN SUCH SOLUTION IS REDUCED TO ZR SPONGE AT THE INTERFACE BETWEEN THE LIQUID MAGNESIUM AND SUCH SUPERATANT SOLUTION. 